What is meant by excretion ? Explain the process of formation of urine.A.. Excretion : Due to metabolism several harmful excretory products are formed, the process of removing toxic waste from the body is called excretion.Formation of urine involves four stages :

Concentration of urine.1) Glomerular filtration:
a) Blood flows from renal artery to glomerulus through afferent arteriole.
b) Filtration of blood occurs in glomerulus.
c) Glomerular filtrate is also known as primary urine which almost equal to blood in chemical composition except the presence of blood cells.2) Tubular Reabsorption : ‘
a) The primary urine passes into proximal convoluted tubule.
b) Useful substances like glucose, amino acids, sodium chloride, potassium ion, bicarbonate ion, and 75% of water are reabsorbed into peritubular network.3) Tubular secretion : .
a) After reabsorption in proximal convoluted region (PCT) the urine travels through the loop of Henle’ into distal convoluted tubule.
b) Here some other wastes like extra salt ions of K+, Na, Cl and H* secretes from peritubular capillaries into distal convoluted tubule which are surrounded by peritubular network.4) Concentration of urine :
a) 75% of water content of the nephric filtrate is reabsorbed in the region of proxi¬mal convoluted tubule and 10% of water passes out of filtrate through osmosis in the area of loop of Henle.
b) Concentration of urine takes place in the area of collecting tubes in the presence of hormone called vasopressin.-The hormone is secreted only when concentrated urine is to be passed out.